Cerebrospinal fluid (CSF)

Cerebrospinal Fluid (CSF) Overview

Cerebrospinal fluid (CSF) is a clear, colorless body fluid found in the brain and spinal cord. It is produced primarily by the choroid plexuses within the ventricles of the brain. CSF circulates through the ventricles, the subarachnoid space surrounding the brain and spinal cord, and the central canal of the spinal cord.

CSF serves several vital functions:

• Buoyancy: Reduces the effective weight of the brain, preventing it from being crushed under its own weight.
• Protection: Acts as a shock absorber, cushioning the brain and spinal cord against trauma.
• Chemical Stability: Provides a stable chemical environment for the central nervous system (CNS).
• Waste Removal: Clears metabolic waste products from the brain.
• Nutrient Transport: May play a role in transporting nutrients.

Analysis of CSF is a critical diagnostic tool for evaluating a wide range of neurological conditions affecting the Central Nervous System (CNS).

CSF Collection (Lumbar Puncture)

CSF is typically obtained via a Lumbar Puncture (LP), commonly known as a spinal tap. This procedure involves inserting a specialized needle into the subarachnoid space in the lower back (lumbar region), usually between the L3/L4 or L4/L5 vertebrae, below the level where the spinal cord ends in adults.

During an LP, several parameters can be assessed, and CSF samples are collected for laboratory analysis:

• Measurement of CSF opening pressure.
• Assessment of CSF appearance (color, clarity).
• Collection of CSF into sterile tubes for various laboratory tests (e.g., cell count, protein, glucose, microbiology, cytology).
• Sometimes used for therapeutic purposes (e.g., injecting medication, draining excess fluid).

Standard CSF Analysis Components

Routine CSF analysis typically includes assessment of physical properties, chemical composition, and microscopic examination.

Appearance & Physical Properties

• Transparency/Turbidity: Normal CSF is crystal clear, like water. Cloudiness (turbidity) indicates an increased number of cells (WBCs, RBCs), microorganisms (bacteria, fungi), or significantly elevated protein levels.

• Color: Normal CSF is colorless. Abnormal coloration can indicate underlying pathology:

  • Pink/Red: Presence of red blood cells (RBCs) due to bleeding into the CSF (subarachnoid hemorrhage) or a traumatic tap.
  • Yellow/Orange/Brown (Xanthochromia): Indicates the presence of bilirubin (from RBC breakdown after hemorrhage), very high protein levels, or melanin (rarely, from metastatic melanoma).
  • Greenish: May indicate severe infection with pus (purulent meningitis) or high bilirubin levels oxidizing to biliverdin.
  • Cloudy/Milky: Suggests high numbers of white blood cells (infection/inflammation), microorganisms, or very high protein.

• Presence of Blood (Erythrocytearchia): RBCs are normally absent. Their presence requires differentiation between a traumatic tap (blood introduced during the LP procedure) and true subarachnoid hemorrhage (SAH) or other CNS bleeding. Characteristics suggesting a traumatic tap include clearing of blood from tube 1 to tube 4 and absence of xanthochromia (after centrifugation). True SAH typically shows consistent blood in all tubes and development of xanthochromia after several hours.

  • CSF appears colorless if RBC count is < ~150/µL.
  • Grayish-pink around 600-1000 RBCs/µL.
  • Pinkish-red around 2000-50,000 RBCs/µL.
  • Visibly bloody > 50,000 RBCs/µL.

  Causes of true blood in CSF include hemorrhagic stroke, ruptured aneurysm, traumatic brain injury, hemorrhage into a tumor, or hemorrhagic encephalitis. RBCs typically clear within days to weeks depending on the cause and severity.

• Xanthochromia: Yellowish discoloration of the CSF supernatant after centrifugation. It's primarily caused by bilirubin released from the breakdown of hemoglobin following bleeding into the subarachnoid space. It usually appears within 2-12 hours after SAH and can persist for 2-4 weeks or longer. Very high protein levels (>150 mg/dL) or systemic hyperbilirubinemia can also cause xanthochromia. "Congestive" xanthochromia may occur with impaired CSF flow (e.g., spinal block due to tumor) leading to protein accumulation and bilirubin leakage.

• Fibrin Clot/Film: Normal CSF does not contain fibrinogen and should not clot. Formation of a fibrin clot or delicate web-like film after collection indicates increased protein levels, particularly fibrinogen, suggesting a significant breakdown of the blood-brain barrier (BBB) or blood contamination. Often seen in tuberculous meningitis, purulent meningitis, spinal block, or CNS tumors.

• Opening Pressure: Measured with a manometer during LP (patient lying on their side). Normal pressure in adults is typically 10-18 cm H₂O (or ~70-180 mm H₂O). Elevated pressure suggests increased intracranial pressure (infection, tumor, hydrocephalus, hemorrhage, idiopathic intracranial hypertension). Low pressure can occur with CSF leaks or dehydration.

• Relative Density (Specific Gravity): Normal range is approximately 1.003-1.008. It increases with higher protein or cell content (e.g., meningitis, uremia). It decreases with increased water content (e.g., hydrocephalus or overhydration).

Chemical Analysis

• Protein Concentration: Normal CSF protein is very low compared to plasma, as the BBB restricts protein entry. Typical lumbar CSF protein is 15-45 mg/dL (0.15-0.45 g/L). Protein levels are lower in ventricular and cisternal CSF.

  • Increased Protein (Hyperproteinarchia): Indicates BBB disruption or increased intrathecal (within the CNS) protein synthesis. Causes include meningitis (bacterial, viral, fungal, tuberculous), encephalitis, subarachnoid hemorrhage, brain tumors (especially near CSF spaces), brain abscess, Guillain-Barré syndrome (albuminocytologic dissociation - high protein, normal cells), multiple sclerosis (due to intrathecal IgG synthesis), spinal block/compression, encephalopathy, traumatic brain injury (TBI), parasitic infections (cysticercosis), arachnoiditis. Approximately 80-85% of CSF protein originates from blood serum (mainly albumin), while ~15-20% is produced intrathecally. The CSF albumin level or the CSF/serum albumin ratio (QAlb) is a specific measure of BBB permeability.
  • Decreased Protein (Hypoproteinarchia): Less common. Can be seen with removal of large CSF volumes, CSF leaks, increased intracranial pressure (sometimes), hyperthyroidism, or sometimes in young children.

• Glucose: Normal CSF glucose is typically about 60% of the simultaneous blood glucose level (approx. 40-70 mg/dL or 2.2-3.9 mmol/L when blood glucose is normal). A blood glucose level should always be measured concurrently for interpretation.

  • Decreased Glucose (Hypoglycorrhachia): CSF glucose < 40 mg/dL or CSF/blood glucose ratio < 0.4-0.5. Suggests increased glucose utilization within the CNS or impaired transport across the BBB. Classic finding in bacterial meningitis. Also seen in tuberculous, fungal, and amebic meningitis; carcinomatous/lymphomatous meningitis; sarcoidosis; parasitic infections; sometimes after subarachnoid hemorrhage. Viral meningitis typically has normal CSF glucose.
  • Increased Glucose (Hyperglycorrhachia): Primarily reflects high blood glucose levels (hyperglycemia) in diabetes mellitus. Rarely elevated independently; may occur slightly with certain types of encephalitis or brain injury.

• Lactate: Reflects anaerobic metabolism within the CNS. Normal CSF lactate is typically < 2.1-2.5 mmol/L. Elevated levels (> 3.5 mmol/L) are highly suggestive of bacterial meningitis (even if Gram stain is negative) and can help differentiate it from viral meningitis (where lactate is usually normal or only slightly elevated). Also elevated in hypoxia, ischemia (stroke), seizures, brain tumors, mitochondrial disease, and certain metabolic disorders.

• pH: Normally tightly regulated around 7.28-7.32. Significant decreases (acidosis) can occur in severe CNS insults like large hemorrhages, severe head injury, extensive infarction, purulent meningitis, or status epilepticus, often indicating poor prognosis. Systemic acidosis/alkalosis usually causes smaller shifts in CSF pH compared to blood pH.

• Ketone Bodies: Normally absent. Their presence (ketonarchia) suggests altered brain metabolism, potentially seen after brain surgery, severe TBI, subarachnoid hemorrhage, or intense CNS stimulation/excitation where ketone utilization may be impaired.

• Nitrites: Not normally present. Their presence indicates the activity of nitrate-reducing bacteria and is therefore suggestive of bacterial meningitis caused by common pathogens like E. coli or Klebsiella. However, many common meningitis pathogens (e.g., Streptococcus pneumoniae, Neisseria meningitidis, Staphylococci, Mycobacterium tuberculosis) do *not* produce nitrites, so a negative test does not rule out bacterial meningitis.

Microscopic Examination (Cell Count & Differential)

Microscopic analysis assesses the number and types of cells present in the CSF.

• Total Cell Count (WBC & RBC): Performed using a hemocytometer chamber. Normal CSF in adults contains very few white blood cells (WBCs) – typically 0-5 lymphocytes/monocytes per microliter (µL or mm³). RBCs should be absent.

  • Pleocytosis: An increased number of WBCs in the CSF, indicating inflammation or infection within the CNS. The degree is often classified:
    • Mild: 6-70 cells/µL
    • Moderate: 70-250 cells/µL
    • Marked (Expressed): 250-1000 cells/µL
    • Severe (Sharply Expressed): >1000 cells/µL
    • Massive: >10,000 cells/µL
  • RBCs are counted to assess for bleeding or traumatic tap.

• WBC Differential: A stained cytocentrifuged preparation (cytospin) is examined microscopically to determine the percentages of different types of WBCs. This is crucial for diagnosing the cause of pleocytosis.

  • Lymphocytes: Predominate in normal CSF (around 70%). Increased lymphocytes (lymphocytic pleocytosis) is characteristic of:

    • Viral meningitis/encephalitis
    • Tuberculous meningitis (often mixed with monocytes)
    • Fungal meningitis
    • Neurosyphilis
    • Multiple sclerosis (mild increase)
    • Parasitic infections (e.g., cysticercosis, toxoplasmosis)
    • Some brain tumors (lymphoma, leukemia)
    • Guillain-Barré syndrome (usually normal or slight increase)
    • Sarcoidosis
    • Autoimmune/inflammatory encephalopathies

  • Monocytes: Make up most of the remaining cells in normal CSF (around 30%). An increase (monocytic pleocytosis), often mixed with lymphocytes, suggests chronic or subacute inflammation:

    • Tuberculous meningitis
    • Neurosyphilis
    • Fungal meningitis
    • Viral meningitis (especially later stages)
    • Brain tumors
    • Multiple sclerosis
    • Chronic inflammatory conditions

  • Macrophages: Derived from monocytes, these phagocytic cells are not normally present. Their appearance indicates response to bleeding (containing hemosiderin or hematoidin pigment after SAH), previous inflammation, infection, or presence of foreign material or necrotic tissue. Often seen after CNS hemorrhage or surgery, or with certain tumors.

  • Neutrophils (Polymorphonuclear Leukocytes - PMNs): Virtually absent in normal adult CSF. A predominance of neutrophils (neutrophilic pleocytosis) strongly suggests acute bacterial meningitis. Also seen in:

    • Early viral meningitis (transiently)
    • Early tuberculous or fungal meningitis
    • Brain abscess
    • Subdural empyema
    • Amebic meningoencephalitis
    • Chemical meningitis (reaction to injected substances)
    • Early phase after hemorrhage (stroke, SAH) or CNS infarction
    • CNS vasculitis
    • Metastatic tumor infiltration

  • Eosinophils: Absent in normal CSF. Eosinophilic pleocytosis (increased eosinophils) is relatively uncommon and suggests:

    • Parasitic infections of the CNS (cysticercosis, toxocariasis, angiostrongyliasis - common cause globally)
    • Fungal infections (especially Coccidioides)
    • Allergic reactions (e.g., to shunts, medications, contrast agents injected into CSF space)
    • Certain types of meningitis (e.g., idiopathic hypereosinophilic syndrome, some drug-induced)
    • Some malignancies (lymphoma, leukemia, brain tumors)
    • Polyarteritis nodosa, sarcoidosis involving CNS

  • Basophils: Extremely rare in CSF, even in disease. May be seen in some chronic inflammatory or allergic conditions or certain leukemias.

  • Plasma Cells: Mature antibody-producing B cells, absent in normal CSF. Their presence indicates chronic inflammation or immune stimulation within the CNS. Found in:

    • Multiple Sclerosis (MS)
    • Subacute Sclerosing Panencephalitis (SSPE)
    • Neurosyphilis
    • Tuberculous meningitis
    • Chronic viral or fungal meningitis/encephalitis
    • Guillain-Barré syndrome (sometimes)
    • Sarcoidosis
    • CNS Lymphoma / Multiple Myeloma involving CNS

  • Immature White Blood Cells (Blasts): Presence of blast cells indicates infiltration of the CNS by leukemia (neuroleukemia), most commonly acute lymphoblastic leukemia (ALL) or acute myeloid leukemia (AML).

  • Other Cells (Arachnoidal, Ependymal): Choroid plexus cells, ependymal cells lining the ventricles, or arachnoidal cells lining the subarachnoid space may occasionally be seen, especially after trauma, surgery, or with certain tumors near CSF spaces.

  • Tumor Cells (Cytology): Microscopic examination specifically looking for malignant cells. Positive cytology indicates primary CNS tumors seeding the CSF (e.g., medulloblastoma, ependymoma, glioma) or metastatic spread to the leptomeninges (leptomeningeal carcinomatosis/lymphomatosis/melanomatosis) from cancers like breast, lung, melanoma, lymphoma, leukemia.

    

    MRI showing leptomeningeal enhancement ("sugar coating") characteristic of leptomeningeal carcinomatosis. CSF cytology is crucial for confirming the presence of tumor cells in such cases.

Microbiological Examination

• Gram Stain: Rapid microscopic examination for bacteria. Sensitivity is limited but crucial in suspected bacterial meningitis.
• Bacterial Culture & Sensitivity: To identify the specific bacteria causing meningitis and determine antibiotic susceptibility.
• Fungal Stain (e.g., India Ink) & Culture: For suspected fungal meningitis (e.g., Cryptococcus).
• Acid-Fast Stain & Culture: For suspected tuberculous meningitis (requires specialized media and longer incubation).
• Viral PCR: Highly sensitive tests for specific viral pathogens (e.g., Herpes Simplex Virus (HSV), Enteroviruses, Varicella Zoster Virus (VZV)).
• Antigen Tests: Rapid tests for specific bacterial or fungal antigens (e.g., Cryptococcal antigen).
• Syphilis Serology (VDRL): Test for neurosyphilis (though often insensitive in CSF).

Lumbar Puncture Procedure Details

• Positioning: Patient typically lies on their side with knees drawn up towards the chest (fetal position) or sits leaning forward over a table to open up the spaces between the lumbar vertebrae.
• Site Preparation: The lower back is cleaned with antiseptic solution and draped. Local anesthetic is injected to numb the skin and deeper tissues.
• Needle Insertion: A special spinal needle is inserted between the spinous processes (usually L3/L4 or L4/L5 space) into the subarachnoid space.
• Pressure Measurement: Once CSF flow is obtained, an opening pressure reading is taken using a manometer if required.
• CSF Collection: CSF is allowed to drip slowly into sterile collection tubes (typically 3-4 tubes are collected for different tests - e.g., tube 1 for chemistry/serology, tube 2 for microbiology, tube 3 for cell count/differential, tube 4 for cell count comparison or special tests).
• Needle Removal & Dressing: The needle is withdrawn, and a sterile dressing is applied.
• Post-Procedure: Patient may be asked to lie flat for a period (e.g., 30-60 minutes) to potentially reduce the risk of post-LP headache, though the benefit of prolonged bed rest is debated. Adequate hydration is encouraged.

Risks and Benefits of Lumbar Puncture

Benefits:

• Provides essential diagnostic information for serious neurological conditions (infections, inflammation, hemorrhage, cancer) that cannot be obtained otherwise.
• Allows measurement of intracranial pressure.
• Can be therapeutic (e.g., removing CSF to lower pressure, administering intrathecal medications).

Risks:

• Post-Lumbar Puncture Headache (PLPH): Most common complication, usually a positional headache (worse when upright, better when lying down) caused by CSF leakage. Typically resolves spontaneously or with conservative measures (fluids, caffeine, rest); rarely requires an epidural blood patch.
• Back Pain: Local pain or discomfort at the puncture site is common but usually temporary.
• Bleeding: Minor bleeding at the site is possible. Significant spinal epidural or subdural hematoma is rare but serious, especially in patients with bleeding disorders or on anticoagulants.
• Infection: Introduction of infection (meningitis) is very rare with proper sterile technique.
• Nerve Injury: Temporary numbness or tingling in a leg can occur if a nerve root is irritated; permanent nerve damage is extremely rare.
• Cerebral Herniation: Extremely rare but potentially fatal complication if LP is performed in a patient with significantly elevated intracranial pressure and a large mass lesion causing pressure differences within the skull. Brain imaging (CT or MRI) is usually performed before LP if increased intracranial pressure or a mass lesion is suspected.

References

1. See, S. J., & Zerah, M. (2007). Cerebrospinal fluid: analysis. In *Encyclopedia of Stress* (Second Edition) (pp. 421-427). Academic Press.
2. Fishman, R. A. (1992). *Cerebrospinal Fluid in Diseases of the Nervous System* (2nd ed.). W.B. Saunders Company.
3. National Institute of Neurological Disorders and Stroke (NINDS). (n.d.). Lumbar Puncture (Spinal Tap). NIH. Retrieved from https://www.ninds.nih.gov/health-information/diagnostic-tests/lumbar-puncture-spinal-tap
4. Mayo Clinic Staff. (n.d.). Spinal tap (lumbar puncture). Mayo Clinic Patient Care & Health Information. Retrieved from https://www.mayoclinic.org/tests-procedures/spinal-tap/about/pac-20395111
5. Deisenhammer, F., Bartos, A., Egg, R., Gilhus, N. E., Giovannoni, G., Rauer, S., & Sellebjerg, F. (2006). EFNS guidelines on routine cerebrospinal fluid analysis. *European Journal of Neurology*, 13(9), 913–922. https://doi.org/10.1111/j.1468-1331.2006.01483.x
6. Lab Tests Online. (n.d.). CSF Analysis. Retrieved from https://labtestsonline.org/tests/csf-analysis